Mirror, mirror on the wall...

Inspired by Herbert Keppler's praise of 500/8 mirror lenses in the August 2003
issue of "Popular Photography" I thought I'd dig my old Tamron 500/8 mirror lens
out of the closet and test its performance against a Canon EF500/4.5 using an EOS
10D 6MP digital DSLR. I wasn't expecting it to do particularly well...

Technical

These tests were done using a Canon EOS 10D body (6MP). The lenses compared
were a Canon EF500/4.5L USM and a Tamron 500/8 mirror lens in a Tamron
adaptall mount. A rare and hard to obtain adaptall mount for the Canon EOS was
used to mount the Tamron lens on the EOS body. Tamron only made a few, so if you
don't have one and you want one, you're probably out of luck. An alternative way
to mount an adaptall lens on an EOS body is to get a Pentax Screw Mount (M42)
Adaptall mount and an M42 -> EOS lens adapter. A Gitzo 1325 tripod with an
Arca-Swiss B1 ball head was used to support the lenses. In each case the lens was
used wide open, a nominal f8 for the mirror lens, f4.5 for the Canon. Since my
experience with 500mm lenses is that 90% of the time you'll be using them wide
open, I think these conditions are appropriate - and for the mirror lens there's
no choice anyway!. "Film" speed was set to ISO 800. In previous tests I've
established that resolution is essentially unaffected by shooting with the 10D at
ISO 800 rather than ISO 100, though of course the noise level is higher.

The Lenses

The Tamron 500/8
SP is, of course, a mirror lens - but that doesn't mean it's all done
with mirrors. In fact the Tamron lens has, according to their literature, 7
elements in 4 groups, only two of which are mirrors. There is a front corrector
lens (which isn't the shape shown in the figure). Most mirror lenses are of
either a Schmidt-Cassegrain design or a Maksutov-Cassegrain design. The
difference is in the shape of the corrector plate. For the Schmidt it's an
aspheric element, for the Maksutov it's a deeply curved spherical meniscus lens.
Looking at the Tamron it appears to be a Schmidt. There's not all that much
theoretical difference in performance - much depends on how well each design is
optimized and built - but the Schmidt is more complex and harder to produce and
often (though not always) signifies a better lens. Really cheap mirror lenses
tend to be Maksutovs since the corrector plate is cheaper to make and the
secondary mirror can be an aluminized spot on the rear of the corrector plate.
The additional small lenses are further corrective lenses and field flattening
lenses (which are sometimes absent in cheaper designs). So although it's a mirror
lens, it has refractive elements too. Theoretically a lens made ONLY of mirrors
has no chromatic aberration, but the addition of the corrective lenses required
to optimize the image quality could (in principle) introduce small amounts of
chromatic aberration.

One Tamron catalog I have refers to the 500/8 using a mangin mirror, which is
a second surface spherical mirror. Normally mirrors used in optics are first
surface, which means the reflective coating is applied to the surface of the
mirror and the light never passes through the glass of which the mirror is made.
In a second surface mirror the light first passes through the glass, is then
reflected from the rear surface and passes through the glass again on
the way out. If the glass is of good optical quality, a mangin (2nd
surface) mirror can be used to correct for spherical aberration and coma
and so one optical component can be used to perform several functions resulting
in improved performance with fewer discrete optical components. The Tamron
mirrors are also stated to be silver coated, which would give them higher
reflectivity for visible light than the more commonly used aluminum mirror
coatings.

The Canon
EF500/4.5L USM lens is a conventional refractive design using one
fluorite element (shown in blue) and one UD glass element (shown in green) to
optimize performance and reduce chromatic aberrations. Note the presence of a
flat element at the front of the lens. This is to protect the fluorite element
which is more susceptible to scratching than conventional glass.

Mirror Optics

Mirror lenses, by virtue of their central obstruction, will
always have somewhat lower contrast than good refractive lenses as the above
figure shows. The ratio of the center obstruction to the full aperture for the
Tamron lens is about 0.44. This means that at, say, 60 lp/mm the MTF will be
about 1/2 that of an unobstructed lens operating at the same f-stop as shown in
the figure below. The figure is applicable to any lens operating at f8. Curve A
would apply equally to a refractive lens like the Canon RF500/4.5L at f8 or to a
mirror lens with no central obstruction operating at f8.

The increase in MTF at high spatial frequencies is real,
but is of no practical benefit in photography for two reasons. First the drop in
MTF at low frequencies will greatly reduce the perceived sharpness of an image.
Second, it's at too high a spatial frequency to be of use since the 10D has a
resolution limit of <70 lp/mm and even on film it will be lost in the
noise.

The out of focus regions of the image will always have that "mirror" look with
harsh detail rather than smooth blurring. Both are consequences of basic optics
and there's no way around these limitations. The lower contrast is due to the
effect of the obstruction on the lens MTF, not due to the use of mirrors as is
sometimes suggested. BTW mirror lenses have the same DOF as refractive lenses of
the same aperture, despite anything you might read to the contrary on some "less
than accurate" websites.

The Tests

I used my trusty old resolution test chart from Edmunds Scientific for the
testing. As you can see, it's seen better days but it's still effective. Shortly
after this test it was caught in a torrential downpour since it was left out in
the garden and a thunderstorm caught it (and me) by surprise - it will soon be
replaced by a brand new copy!

500mm lens view

500mm lens + 1.4x TC view

Above is the field of view of the 500mm lens and 500mm lens + 1.4x Tamron TC.
I'm showing this so you can get an idea of where the enlarged segments of the
images which are shown below were taken from. The red squares show the
approximate locations of the samples. In each case below the Tamron 500/8 mirror
lens sample is on the left and the Canon EF500/4.5L sample is on the right,

Note: All the image presented below are
directly from the camera. No sharpening or other manipulation has been done,
other than cropping unless otherwise noted

The Results

Image Center - No TC used - 200%

Tamron 500mm mirror

Canon EF 500/4.5L

Well, here's a bit of a surprise. Not all THAT much difference. Sure the Canon
lens is better but not by as much as you'd expect. Now I KNOW for sure that the
Canon lens is very, very sharp. In fact 500mm lenses don't come any sharper. What
we're seeing here is something of the limits of digital resolution. The 10D
sensor just can't resolve more than about 60 lp/mm - and before you suggest the
same test using a 1Ds, remember that the pixel DENSITY (hence resolution) is
higher on a 10D than a 1Ds - the 1Ds just more pixels but they are spread over a
wider field, so the 1Ds pixel pitch is actually larger (8.8 microns on the
1Ds vs. 7.4 microns on the 10D).

Image ~ 70% of the distance to the corner. No TC -
200%

Tamron 500mm mirror

Canon EF500/4.5L

In this case the difference is quite a bit more significant. The
Canon lens is sharper, no doubt about it. Note that we're not all the way out to
the corner either. So now let's add a 1.4xTC and see what happens

Image center - 500mm lens + 1.4x TC - 200%

Tamron 500mm mirror + Tamron 1.4x TC

Canon EF 500/4.5L + Tamron 1.4x TC

Actually the first thing that happens is that the Tamron lens and 1.4x TC
won't work on the 10D! Manual focus lens mounts (and lens adapters) are different
from AF mounts in that they don't contact a microswitch in the lens mount of the
10D (or any EOS body) which tells the camera to look for an AF lens. The Tamron
TC is autofocus, so it trips this microswitch, which is fine as long as you
then attach and AF lens. If you attach a manual focus lens, the body still looks
for an AF lens and when it doesn't find one it generates an error condition. The
solution is to rotate the Tamron TC slightly so it's not locked into place. This
untrips the microswitch and all is well with MF lenses. Just beware of the lens
accidentally falling off!

When it comes to performance, again we can see the Canon lens is clearly
better. Not a big surprise. A bigger surprise is that the mirror lens holds up as
well as it does.

Image edge - 500mm lens + 1.4x TC - 200%

Tamron 500mm mirror + Tamron 1.4x TC

Canon EF 500/4.5L + Tamron 1.4x TC

Well, not much difference here. Both show some chromatic
aberration, probably due to the TC as much as the lens. The Canon less is very
well chromatically corrected and the mirror lens should have no intrinsic
chromatic aberration. Though the Canon lens is at f4.5, and so would be expected
to show more background blur than the 500mm f8 lens, this is still a valid
comparison since normally you'd be shooting with the 500mm lens wide open.

Bokeh of 500mm lenses (no TC) - 100%

Tamron 500/8 mirror

Canon EF500/4.5L

Here we have a clear, no doubt, hands down winner, just as we would expect.
The mirror lens produces a very confused background with lots of false detail due
to effect of the central obstruction inherent in the design of all photographic
mirror lenses. The conventional refractive lens produced a much smoother out of
focus region.

Macro

The Tamron focus down to 1.7m (1:3) while the Canon only focuses to 5m (1:9) -
however performance does drop when the Tamron is used in the Macro mode, thought
it may still be acceptable. It's also fixed at f8, so you can't stop down for
more DOF and you have the typical mirror lens "bokeh" (distracting background
blur) to contend with. Not an ideal macro lens, though certainly handy. Getting
the EF500/4.5L to focus down to 1.7m would need an awful lot of extension
tubes!

Tamron 500/8 focused at 1.7m (1:3) Image downsized to 600x400
pixels

100% crop from original, sharpened and levels adjusted

Other Stuff

The Tamron lens does seem to have one interesting property, and that's that
maybe you can get away with shooting (static subjects) at slow shutter speeds
with the lens on a tripod without using MLU (Mirror Lock Up)and/or a remote
release. The lower mass of the lens and the smaller size makes it less
susceptible to "wobble" induced by shaky hands! In a test I took 4 images at 1/4s
shutter speed. Both the Tamron mirror lens shots were sharp. One of the Canon
shots was almost as sharp, but one was clearly blurred by camera induced
movement. Maybe I could have used ISO 100 instead of ISO 800 for all these tests
and shot everything at 1/8s instead of 1/60s!

I think one reason for this behavior is that with the Tamron
500/8 you can (indeed have to) mount the camera on the tripod rather than the
lens. With the EF 500/4.5L you have to mount the lens on the tripod, which puts
the camera body about 9" from the pivot point of the (locked down) ballhead. Now
if you push left or right on the camera with, say, 1lb of force and it's 9" from
the support point you put 0.75 ft-lbs of torque on the system and you get quite a
deflection. When you let go that stored energy appears in the form of vibrations.
If you push with the same amount of force on the camera and the camera is
directly over the (locked down) support point, you put no torque on the support
and the deflection is zero.When you let go, nothing much happens. If you bang the
end of the EF500/4.5L you'll see the shake through the viewfinder go on for a
second or so. If you bang the end of the 500/8 you won't see any shake at all
(except maybe from the direct impact if you hit it hard enough!). Note that this
is on a Gitzo 1325 Carbon fiber tripod (legs not extended) and using an
Arca-Swiss B-1 ball head, so the tripod support isn't by any means flimsy.

Verdict?

I declare the Canon EF500/4L the winner!!

However keep in mind that the price of a new Tamron 500/8 lens is
$400, while a used Canon EF500/4.5L will probably cost you around $3000
- and its replacement, a new EF500/4L IS, will cost you at least double that. The
Tamron weighs about 19oz, while the 500/4.5L weighs 6.6lbs and the 500/4L IS
weighs about 8.5lbs. The 500/4L IS and 400/4.5L are just over 15" long, while the
Tamron 500/8 is about 3.6" long. The Tamron gives you 1:3 macro, the Canon
1:9.

The Canon lens is also autofocus and f4.5. The Tamron lens is manual focus and
nominally f8, though closer to f9 in practice. That's two stops plus AF - a
huge advantage for the Canon lens in practical field use. In fact to
shoot the images above I used ISO 800 in order to keep shutter speeds up above
1/60s with the Tamron lens to make sure that sharpness wasn't affected by camera
movement (though this might not have been necessary in light of the observations
above - but it never hurts to be safe). Focusing on the Tamron 500/8 mirror lens
is very smooth, but very sensitive. You're also at f8, so the viewfinder isn't as
bright and the extra DOF makes deciding when something is in focus a bit tricky.
You have to rock focus back and forth a few times to make sure you've got good
focus. You need a pretty delicate touch to optimize focus. If your subject is
moving you're probably going to have a tough time tracking it with focus, in fact
I'd go as far as to say that attempting to track a moving subject with a manual
focus 500mm mirror lens is pretty much doomed to failure. Even following focus a
bird hopping around or an animal grazing and constantly moving it's head could be
tricky. Forget about flying birds.

The Canon lens also has a rotating tripod collar, so it's much easier to
rapidly switch from horizontal to vertical format than with the Tamron 500/8 lens
which doesn't have a tripod collar and so is supported on the camera. Some mirror
lenses (but not the cheap ones!), do have a rotating tripod collar

So how much of the apparent equality in optical performance is due to the use
of digital rather than film? Probably quite a bit, however if you are shooting
digital you probably won't care, since digital performance is what you are
interested in. I've previously tested these lenses using Velvia and I saw a
greater difference (obviously with the Canon lens still being the winner!).

Should you buy a mirror lens?

Well I used to say no, it's not a good idea. I still don't think they are
great, but based on this test I'd say that if you're shooting with a 10D and your
budget is limited, a mirror lens might be worth considering. However note that a
$60 "no-name" mirror lens may be significantly worse than a $400 Tamron 500/8 SP
which does have a reputation of being one of the better mirror lenses. The Nikon
500/8 is reputed to be even better, but the gray version currently sells for $630
and the US version for $860, which makes it less attractive. I suspect the Contax
Zeiss Mirotar T*500/8, which has a street price around $1800, is better still.
Even in mirror lenses, you usually get what you pay for. For not all that much
more than the price of these "better" mirror lenses (and significantly less than
the Contax) there are a number of conventional 500mm lenses available, such as
the Sigma 50-500 ($900) and 170-500 ($600), which not only give you zoom
capability, but autofocus and aperture control as well, in addition to a stop or
so of extra speed.

The manual focus fixed-aperture mirror lens can't be beaten on size, weight or
cost, but it's certainly second in terms of convenience and overall performance
when compared to autofocus refractive lenses with variable aperture. If I was
going to attempt to climb Mt Everest or run a marathon carrying a 500mm lens, I'd
go for a mirror. If I was planning to visit Yellowstone in the fall, I'd still
drag my EF500/4.5L along with me. On a limited budget, it's a tough call. A
mirror lens may be the best you can do if you really need 500mm and you only have
a few hundred dollars to spend. There are under $200 500mm f8 refractive lenses
sold. These are usually fairly simple 4 element lens designs. None of the major
lens manufacturers make them, but they show up from time to time under "house
brands" or names you don't recognize. Again it's a 500mm lens and it costs under
$200. Don't expect miracles. They are usually much longer than a mirror lens and
their optical performance isn't great, so whether you go for a cheap mirror lens
or a cheap refractive lens is more a matter of convenience than optics.

The other thing to consider is whether you actually need 500mm? If you're
shooting with a 10D, a 500mm lens will give you the same field of view as an
800mm lens on a full frame 35mm camera (due the the 1.6x "multiplying" factor -
which is really a 1,6x cropping factor). An 800mm lens is pretty long - in
fact longer than most serious full frame 35mm nature photographers use.. A 300mm
lens on a 10D gives the same field of view as a 480mm lens on a full frame 35mm
camera and makes a very good general purpose telephoto. You can get 100-300
zooms, or if you want the ultimate, a 300/4L IS lens. With 1.4x TC a 300mm on a
10D gives you the equivalent view of a 672mm lens on a full frame 35mm camera.
Something to think about anyway.

Which Mirror Lens?

Well, that depends on your budget and which camera you are using. If you
really want a mirror lens, here are a few choices:

Readers' Comments

Hi Bob; I use the same Edmund 24x36" targets sometimes. One I have mounted on 32x40" rigid foam core; in the center....Around the edges; and in the blank white areas; are added other NON standard things to check lenses with.....Some areas have some scraps of 7.5 minute detailed USGS topo maps; different point sizes of text; canned test charts that are from our color copier; line artwork of human faces; and a piece of a dollar bill...........Another interesting thing to add is a lower contrast version of the USAF chart....This really drops the resolution numbers; closer to really world objects....Another nice addition is a scale; in inches and millimeters.....This is a good double check for the actual magnification; which is needed to calculate the line pairs/mm....In a couple of decades; sometimes a goof is made; the scale is always true...Mine also has a data area; for postit notes; or 3x5 cards; to sharpie in the aperture; lens and shutter speed.......Regards; Kelly

When facing the decision to buy a really expensive real telephoto or a cheap mirror I went for a really cheap mirror, the russian rubinar 500mm/5,6 for at that time less than $200. Although others might argue differently I wanted to spend as little as possible for a suboptimal lens and te rubinar fit the bill perfectly for me. I try to attach a JPG with a quick and dirty comparison to my Planar 100/2.0. I basically came to same conclusion as Bob: Mirrors are behind but not as far as one might think and the real problem are the donuts. And I have some shots that I wouldn't have gotten other wise (http://www.photo.net/photodb/photo?photo_id=1193502&size=lg), so for me it paid off.

Here's a hand held macro shot using the Tamron 500/8. Of course I had to use a high speed setting but after cropping and processing to reduce noise and reducing size for the web, you can't tell! Not sure a 12" x 12" print would look as good.

There is one more aspect concerning mirror lenses. Bokeh of highlights and light-sources turns into aweful (IMHO) circular rings. Other than that and being very slow, they're more than any normal shooter would need, considering that most of us don't lug around a 500/4 lens everywhere we go.

"By any chance, any idea how the "old" Vivitar 600m f/8 and 800mm f/11 Solid Cats and the Vivitar Series 1 450mm f/4.5 fare in this context?"

Absolutely none, sorry.

We may not all lug a 500/4 around with us all the time, but some of us do lug a 500/4 around quite a bit of the time, and most of the time that we don't, we wish we had it with us (but carried by someone else of course).

A trick that I saw used at Perkins Observatory (in Ohio) with their _big_ Cassegrain lens is to create a mask covering the front of the telescope with a circular hole cut to the side of it so the diameter of the circle was from the edge of the telescope to the edge of the center obstruction. This should have the effects of stopping down the lens (on the tamron lens described, with the largest unobstructed circle you get f/29, I don't know if you'd want much smaller of an aperture than that), create a more "normal" bokeh and should produce somewhat greater contrast and sharpness (this last bit was their stated goal, they didn't much care about bokeh). This makes it almost unusable for anthing other than static subjects but it should be more usable for most macro subjects.

Off axis aperture masks won't work very well on a 500/8 mirror lens, mainly beacuse of the abysmally small aperture which results. The small aperture not only means long shutter speeds (or high ISO) but diffraction will kill off sharpness and contrast even more than the central obstruction does.

It works for telescopes (and I've used it myself) because the central obstruction isn't usually so large and large f-ratios aren't so important since you can compensate with longer focal length eyepieces. There are also issues of looking through a smaller "slug" of turbulant air and see "seeing" can improve even though resolution drops.

You could make an unobstructed mirror lens, and I've seen designs for some, but the complexity of the optics required to get a good image and the added mechanical design required means they'd probably cost as much as a refractive lens, plus they'd be significantly larger than conventional cassegrain based mirror lenses.

Coming from an amateur astronomy background, it has always been my understanding that Maksutov designs tend to be more expensive than Schmidt ones. Creation of Schmidt type telescopes is more complex and costly up to a point - but larger aperture Maksutovs are very expensive to manufacture since their giant meniscus lens requires so much more glass, grinding, and polishing time than an equivalent size corrector lens in a Schmidt design.

Also, the large secondary obstruction in the Schmidt designs tends to leave images less contrasty than Maksutov ones which would seem to be a strong reason to favor Maksutov designs in photographic applications.

I think Schmidt designs are more popular because they are usually a few stops faster (which is a huge consideration in photography, less so in astrophotography where Maksutov scopes form a healthy minority).

Real-world use of a quality 500mm mirror lens, such as my first gen Nikkor Reflex F8 that I've used since 1976, is not a negative in relation to a regular lens design. I sold my first Formula 1 shot to Road & Track back in 1978 with my trusty mirror lens, and sold many more over the years with it too. Here's a scan of a well-worn tearsheet of the shot (Nikon F2S/500mm Reflex/K64):

http://www.jaypix.com/pix/villb01.jpg

It was only years later that I learned the 500mm Reflex was too slow, too soft, had a hot-spot, couldn't be hand-held and the viewfinder was too dark.

The effect of the secondary obstruction is really related only to its size relative to the primary mirror. For a given aperture and focal length I don't think there's any significant difference between Schmidt and Maksutov designs. The Maksutovs can be cheaper because you can I think (with the right design) just silver a small spot on the back of the corrector plate rather than having to fabricate and use a second mirror and somehow fix it to the Schmidt corrector. Also I believe tha Makstov design uses a corrector with all spherical surfaces, which can be easier (thus cheaper?) to make then the complex aspheric corrector used by Schmidts.

In the end it probably doesn't matter much which design is used. It's probably the care taken in manufacture and the accuracy of the surfaces which are the most important factors in determining the image quality produced by the lens.

I've been extremely satisfied with my Nikkor 500 F/8. I generally shoot it hand-held (which theoretically should result is fuzzy images but in practive doesn't). It's light weight, reliable, and is sharper than some of my other lenses with TC's hooked on. I'm sure that a real 500mm glass lens made my Nikon will be sharper but I'd still have to carry the blasted thing around.

My main complaint with the lens is related to the tripod collar. It's small and very close to the base of the lens. This makes it useless on My D1x and F5 since they have the extended grip. If using a tripod collar is important to you, many mirror lenses won't be helpful.

My Tamrom mirror lens shows much nicer out of focus blurr (= bouquet or "bokeh" as phnetters tend to bastardize this word ..), when the out of focus stuff is in FRONT of the focal - plane. It admittedly gets kind of kinky when I focus in front of a coarsely structured background such as trees, houses, ... .

Secondly, I have had really nice results in sky/moon shots with this lens. I was going to give it away, when the lunar ecclipse came around several weeks ago. I am so impressed with its pictures in "astronomy" that I have decided to keep it and experiment instead.

I purchased a Phoenix / Samyang 500mm f8 Mirror lens brand new for $98.00 I think this is the cheapest one on the market in the US. I am a beginning photographer and have shot four rolls of film with this lens so far. It can take pleasing photos if you understand its limitations and practice using it properly. Here are two photos taken with this lens:

As you can see, the lens is capable of a reasonable degree of sharpness. The limited depth of field is evident in these photos.

I am sure that with experience, I will take better shots with this lens. For those of us spoiled by "auto-everything" cameras it will take a bit of patience to master a 500mm f8 lens. Here are a few guidelines for this lens. Use at the minimum, 200asa film on bright sunny days for easier focusing. That will give you a shutter speed of 1/500 with the lens fixed f8 aperture on a sunny day. Always compose knowing that you have a very limited depth of field. Also, keep your backgrounds as simple as possible to minimize the donut shaped highlights. Finally, have fun learning to master it!

This interesting article was just pointed out to me.... by the person to whom I sold my Tamron 500/8! Perhaps I should have kept it!

The reason I'm commenting here, though, is that I am curious if anyone else has ever physically compared the Nikkor 500/8 alongside the Tamron 500/8. I had the opportunity to do this closely a couple years ago and I swear the two came off the same assemly line, with only minor cosmetic differences such as a different focusing ring grip. I seem to redall the specifications of the two lenses seemed identical, too. I've not had the opportunity to compare images from the two, side by side.

One caveat I'd add for using the Tamron 500/8 or any other mirror lens is that you will be tempted to try to handhold the lens, due to the compact size and light weight. Don't! Always try to use at least a monopod, or better yet, a tripod. Your results will be worth it.

Over the years, I got many good shots with the Tamron, only reluctantly sold it because my main user cameras are now EOS (see comments above, about the very elusive Adaptall-2 for EOS) and I have other lenses providing the same focal length.

I am aware of the fact that I am a "bit" late entering this discussion but,
any lens has it's uses if not virtues!
I do not disagree with any of the stated disadvantages of catadioptric lenses BUT:

I use an ancient MTO 1100mm/f10.5 lens which, proves to be very hard to handle most of the times.
However, when responding to the question "which lenses do you use, and why", I went out and tried to show why I use the 3 KG, 7 lbs MTO.
One set to provide an idea is here:
http://www.flickr.com/photos/bartpeeren/sets/72157602912301914/
Pentax Ist-DL with 50/2.8 AF macro and EOS 350D with the MTO from same position, about 1/4 mile/400meters.
A second, to show the bokeh does not always distract is here:
http://www.flickr.com/photos/bartpeeren/136006122/